System and process for recovering products using simulated-moving-bed adsorption

ABSTRACT

A process according to various approaches includes flushing an intermediate transfer line at first flow rate during a first portion of the step-time interval. The process also includes flushing the intermediate transfer line at as second different flow rate during a second portion of the step-time interval so that a greater volume of fluid is flushed from the intermediate transfer line during one of the first portion and the second portion of the step-time interval than during the other of the first portion and the second portion of the step-time interval.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No.61/570,948 which was filed on Dec. 15, 2011.

FIELD OF THE INVENTION

The subject invention relates to a process for the adsorptive separationof a preferentially adsorbed component from a feed stream. Morespecifically, the invention relates to a process for the continuoussimulated countercurrent adsorptive separation of aromatic hydrocarbons.

BACKGROUND OF THE INVENTION

Para-xylene and meta-xylene are important raw materials in the chemicaland fiber industries. Terephthalic acid derived from para-xylene is usedto produce polyester fabrics and other articles which are in wide usetoday. Meta-xylene is a raw material for the manufacture of a number ofuseful products including insecticides and isophthalic acid. One or acombination of adsorptive separation, crystallization and fractionaldistillation have been used to obtain these xylene isomers, withadsorptive separation capturing a great majority of the market share ofnewly constructed plants for the dominant para-xylene isomer.

Processes for adsorptive separation are widely described in theliterature. For example, a general description directed to the recoveryof para-xylene was presented at page 70 of the September 1970 edition ofChemical Engineering Progress (Vol. 66, No 9). There is a long historyof available references describing useful adsorbents and desorbents,mechanical parts of a simulated moving-bed system including rotaryvalves for distributing liquid flows, the internals of the adsorbentchambers and control systems. The principle of using a simulated movingbed to continuously separate the components of a fluid mixture bycontact with a solid adsorbent is as set forth in U.S. Pat. No.2,985,589. U.S. Pat. No. 3,997,620 applies the principle of thesimulated moving bed to the recovery of para-xylene from a feed streamcontaining C₈ aromatics, and U.S. Pat. No. 4,326,092 teaches meta-xylenerecovery from a C₈-aromatics stream.

Adsorptive separation units processing C₈ aromatics generally use asimulated countercurrent movement of the adsorbent and the feed stream.This simulation is performed using established commercial technologywherein the adsorbent is held in place in one or more cylindricaladsorbent chambers and the positions at which the streams involved inthe process enter and leave the chambers are slowly shifted along thelength of the beds. A typical adsorptive separation unit is illustratedin FIG. 8 and includes at least four streams (feed, desorbent, extractand raffinate) employed in this procedure and the location at which thefeed and desorbent streams enter the chamber and the extract andraffinate streams leave the chamber are simultaneously shifted in thesame direction at set intervals. Each shift in location of the transferpoints delivers or removes liquid to or from a different bed within thechamber. In general, to simulate countercurrent movement of theadsorbent relative to the fluid stream within the chamber, the streamsare shifted in the general direction of fluid flow, i.e. the downstreamdirection, within the chamber to simulate the solid adsorbent moving inthe opposite, i.e. upstream, direction. The lines at these transferpoints are reused as each stream enters or leaves the associated bed,and each line therefore carries one of the four process streams duringsome point of the cycle.

The art recognizes that the presence of residual compounds in thetransfer lines can have detrimental effects on a simulated-moving-bedprocess. U.S. Pat. Nos. 3,201,491; 5,750,820; 5,884,777; 6,004,518; and6,149,874 teach the flushing of the line used to deliver the feed streamto the adsorbent chamber as a means to increase the purity of therecovered extract or sorbate component. Such flushing avoidscontamination of the extract stream with raffinate components of thefeed remaining in this line when it is subsequently used to withdraw theextract stream from the chamber. U.S. Pat. No. 5,912,395 teachesflushing of the line just used to remove the raffinate stream in orderto avoid contaminating feed with raffinate when this line is used todeliver the feed stream to the adsorbent chamber. All of thesereferences teach flushing such lines back into the adsorbent chamber,thus increasing the separation load within the chamber. U.S. Pat. No.7,208,651 discloses flushing away from the adsorbent chamber thecontents of a transfer line which previously has been used to remove theraffinate stream with one or both of a feed mixture and a materialwithdrawn from the adsorption zone. The residual raffinate within thetransfer line is flushed to join the raffinate stream as feed to araffinate column. U.S. Pat. No. 6,149,874 discloses flushing residualfeed from a common section of fluid distribution piping to a boostercircuit.

One previous exemplary system utilized up to three flushes to handleresidual fluid remaining in the transfer lines. A primary flushdisplaced residual extract from the transfer line just used to removethe extract stream with fluid from the desorption zone of the chamberjust below the desorbent stream and directed it through a rotary valveto a transfer line just used to inject the feed stream. Because thevolumes in the transfer lines were about equal, theextract-plus-desorbent fluid displaced the residual feed that hadpreviously been in the transfer line into the adsorbent chamber justabove the current feed stream position so that the residual feed couldbe separated with the feed stream within the adsorptive separationchamber and to avoid contamination of the extract stream with theresidual feed remaining in the transfer line when the extract streamsubsequently shifted to the transfer line previously occupied by thefeed stream. Further, the residual extract from the primary flush usedto displace the feed remained in the transfer line to be subsequentlywithdrawn by the extract stream to increase yield of the extractproduct.

The exemplary system sometimes included a secondary flush. The secondaryflush utilized a flush of fluid, typically desorbent, through thetransfer line and into the chamber immediately below the extract line.The secondary flush provided a “wash” of this transfer line with thedesorbent to minimize the amount of contaminates, including raffinate,feed, and other components that may remain in the transfer line afterthe primary flush so that these materials were not withdrawn from thetransfer line with the extract. Because this transfer line waspreviously flushed with desorbent and extract via the primary flush, thesecondary flush was typically used in applications requiring high purityextract. The secondary flush would push the extract and desorbentmaterial previously in the transfer line back into the adsorptiveseparation chamber. The secondary flush is an optional flush utilized tomeet high purity demands of the extract product.

In some systems, a tertiary flush was also utilized. The tertiary flushincluded a flush of the transfer line previously occupied by theraffinate withdrawal stream. The tertiary flush was utilized to removethe residual raffinate from this transfer line to restrict thisraffinate from being injected back into the adsorbent chamber with thefeed upon subsequent arrival of the feed stream to the transfer line.Because the raffinate stream is depleted of the desired extractcomponent, the tertiary flush was carried out so that the residualraffinate was not injected back into the adsorptive separation chamber,which would otherwise increase the separation demands in order to removethis additional raffinate material. The tertiary flush was accomplishedby flushing the transfer line away from the adsorptive separationchamber with fluid from a port of the chamber adjacent to the transferline.

SUMMARY OF THE INVENTION

According to various approaches, a process is provided for separatingcomponents in a feed stream by simulated countercurrent adsorptiveseparation. The process includes introducing a feed stream and adesorbent stream into two different ports via two differentcorresponding transfer lines along a multi-bed adsorptive separationchamber. The feed stream has at least one preferentially adsorbedcomponent and at least one non-preferentially adsorbed component. Themulti-bed adsorptive separation chamber has plurality of beds that areserially connected in fluid communication and comprising a predeterminednumber of spaced ports with corresponding transfer lines in fluidcommunication therewith for introducing and removing fluid into and fromthe adsorptive separation chamber. The process also includes withdrawingan extract stream and raffinate stream through two different ports ofthe multi-bed adsorptive separation chamber via two differentcorresponding transfer lines. The process according to this approachincludes flushing an intermediate transfer line at first flow rateduring a first portion of the step-time interval. The process alsoincludes flushing the intermediate transfer line at as second differentflow rate during a second portion of the step-time interval so that agreater volume of fluid is flushed from the intermediate transfer lineduring one of the first portion and the second portion of the step-timeinterval than during the other of the first portion and the secondportion of the step-time interval.

According to one approach, the flow rate is a stepped flow rate and thefirst flow rate is a first constant flow rate and the second flow rateis a second constant flow rate. According to another approach, at leasta portion of one or both of the first flow rate and the second flow rateis ramped and changes over time.

According to another approach, a process is provided for the separationof components in a feed stream comprising at least one preferentiallyadsorbed component and at least one non-preferentially adsorbedcomponent by simulated countercurrent adsorptive separation thatintroducing a feed stream into a port of a multi-bed adsorptiveseparation chamber comprising a plurality of ports with correspondingtransfer lines via one transfer line in fluid communication with theport. The process also includes withdrawing an extract stream from theadsorptive separation chamber through the one transfer line, wherein theextract stream has a higher concentration of the preferentially adsorbedcomponent than the feed stream and a lower concentration of thenon-preferentially adsorbed component than the feed stream. The processaccording to this approach further includes introducing a desorbentstream into the port of the adsorptive separation chamber. The processalso includes withdrawing an extract stream from the adsorptiveseparation chamber, wherein a portion of each stream introduced andwithdrawn through the transfer line remains in the transfer line asresidual fluid. In addition, the process includes flushing the transferline at first flow rate for a first portion of a step-time intervalduring which the transfer line is not occupied by one of the feedstream, the extract stream, the desorbent stream, and the raffinatestream. Further the process includes flushing the intermediate transferline at as second different flow rate for a second portion of thestep-time interval so that a greater volume of fluid is flushed from thetransfer line during one portion of the step-time interval than theother portion of the step-time interval

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 2 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 3 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 4 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 5 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 6 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 7 is a simplified diagram of a simulated-moving-bed adsorptionprocess in accordance with various embodiments of the invention;

FIG. 8 is a compositional diagram of fluid within a simulated-moving-bedadsorptive separation chamber in accordance with various embodiments ofthe invention;

FIG. 9 is a perspective view of a rotary valve in accordance withvarious embodiments of the invention;

FIGS. 10-12 are graphs illustrating the volumetric flow rate of fluidthrough transfer lines in accordance with various embodiments of theinvention; and

FIG. 13 is a simplified diagram of a Prior Art simulated-moving-bedadsorption process.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments of the present invention. It will further beappreciated that certain actions and/or steps may be described in aparticular order of occurrence while those skilled in the art willunderstand that such specificity with respect to sequence is notactually required. It will also be understood that the terms andexpressions used herein have the ordinary technical meaning as isaccorded to such terms and expressions by persons skilled in thetechnical field as set forth above except where different specificmeanings have otherwise been set forth herein.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

Adsorptive separation is applied to the recovery of a variety ofhydrocarbon and other chemical products. Chemical separations using thisapproach which have been disclosed include the separation of mixtures ofaromatics into specific aromatic isomers, of linear from nonlinearaliphatic and olefinic hydrocarbons, of either paraffins or aromaticsfrom a feed mixture comprising both aromatics and paraffins, of chiralcompounds for use in pharmaceuticals and fine chemicals, of oxygenatessuch as alcohols and ethers, and of carbohydrates such as sugars.Aromatics separations include mixtures of dialkyl-substituted monocyclicaromatics and of dimethyl naphthalenes. A major commercial application,which forms the focus of the prior references and of the followingdescription of the present invention without so limiting it, is therecovery of para-xylene and/or meta-xylene from mixtures of C₈aromatics, due to typically high purity requirements for these products.Such C₈ aromatics usually are derived within an aromatics complex by thecatalytic reforming of naphtha followed by extraction and fractionation,or by transalkylation or isomerization of aromatics-rich streams in suchcomplexes; the C₈ aromatics generally comprise a mixture of xyleneisomers and ethylbenzene. Processing of C₈ aromatics usingsimulated-moving-bed adsorption generally is directed to the recovery ofhigh-purity para-xylene or high-purity meta-xylene; high purity usuallyis defined as at least 99.5 wt.-% of the desired product, and preferablyat least 99.7 wt.-%. It should be understood, that while the followingdetailed description focuses on the recovery of high-purity para-xylenefrom a mixed xylene and ethylbenzene stream, the invention is not solimited, and is also applicable for separating other components from astream comprising two or more components. As used herein, the termpreferentially adsorbed component refers to a component or components ofa feed stream that are more preferentially adsorbed than one or morenon-preferentially adsorbed components of the feed stream.

The invention normally is employed in an adsorptive separation processwhich simulates countercurrent movement of the adsorbent and surroundingliquid as described above, but it may also be practiced in a cocurrentcontinuous process, like that disclosed in U.S. Pat. No. 4,402,832 andU.S. Pat. No. 4,478,721. The functions and properties of adsorbents anddesorbents in the chromatographic separation of liquid components arewell-known, and reference may be made to U.S. Pat. No. 4,642,397, whichis incorporated herein, for additional description of these adsorptionfundamentals. Countercurrent moving-bed or simulated-moving-bedcountercurrent flow systems have a much greater separation efficiencyfor such separations than fixed-bed systems, as adsorption anddesorption operations are continuously taking place with a continuousfeed stream and continuous production of extract and raffinate. Athorough explanation of simulated-moving-bed processes is given in theAdsorptive Separation section of the Kirk-Othmer Encyclopedia ofChemical Technology at page 563.

FIG. 1 is a schematic diagram of a simulated-moving-bed adsorptionprocess in accordance with one aspect. The process sequentially contactsa feed stream 5 with adsorbent contained in the vessels and a desorbentstream 10 to separate an extract stream 15 and a raffinate stream 20. Inthe simulated-moving-bed countercurrent flow system, progressiveshifting of multiple liquid feed and product access points or ports 25down an adsorbent chamber 100 and 105 simulate the upward movement ofadsorbent contained in the chamber. The adsorbent in asimulated-moving-bed adsorption process is contained in multiple beds inone or more vessels or chambers; two chambers 100 and 105 in series areshown in FIG. 1, although a single chamber 902 as illustrated in FIG. 13or other numbers of chambers in series may be used. Each vessel 100 and105 contains multiple beds of adsorbent in processing spaces. Each ofthe vessels has a number of ports 25 relating to the number of beds ofadsorbent, and the position of the feed stream 5, desorbent stream 10,extract stream 15 and raffinate stream 20 are shifted along the ports 25to simulate a moving adsorbent bed. Circulating liquid comprisingdesorbent, extract and raffinate circulates through the chambers throughpumps 110 and 115, respectively. Systems to control the flow ofcirculating liquid are described in U.S. Pat. No. 5,595,665, but theparticulars of such systems are not essential to the present invention.A rotary disc type valve 300, as characterized for example in U.S. Pat.No. 3,040,777 and U.S. Pat. No. 3,422,848, effects the shifting of thestreams along the adsorbent chamber to simulate countercurrent flow.Although the rotary disc valve 300 is described herein, other systemsand apparatus for shifting the streams along the adsorbent chamber arealso contemplated herein, including systems utilizing multiple valves tocontrol the flow of the streams to and from the adsorbent chamber 100and/or 105 as for example, described in U.S. Pat. No. 6,149,874.

Referring to FIG. 9, a simplified exploded diagram of an exemplaryrotary valve 300 for use in an adsorptive separation system and processis depicted. A base plate 474 includes a number of ports 476. The numberof ports 476 equal the total number of transfer lines on the chamber(s).The base plate 474 also includes a number of tracks 478. The number oftracks 478 equal the number of net input, output, and flush lines forthe adsorptive separation unit (not shown in FIG. 9). The net inputs,outputs, and flush lines are each in fluid communication with adedicated track 478. Crossover lines 470 place a given track 478 influid communication with a given port 476. In one example, the netinputs include a feed input and a desorbent input, the net outputsinclude an extract output and a raffinate output, and the flush linesinclude between about one and about four flush lines. As the rotor 480rotates as indicated each track 478 is placed in fluid communicationwith the next successive port 476 by crossover line 470. A seal sheet472 is also provided.

The various streams involved in simulated-moving-bed adsorption asillustrated in the figures and discussed further below with regard tothe various aspects of the invention described herein may becharacterized as follows. A “feed stream” is a mixture containing one ormore extract components or preferentially adsorbed components and one ormore raffinate components or non-preferentially adsorbed components tobe separated by the process. The “extract stream” comprises the extractcomponent, usually the desired product, which is more selectively orpreferentially adsorbed by the adsorbent. The “raffinate stream”comprises one or more raffinate components which are less selectivelyadsorbed or non-preferentially adsorbed. “Desorbent” refers to amaterial capable of desorbing an extract component, which generally isinert to the components of the feed stream and easily separable fromboth the extract and the raffinate, for example, via distillation.

The extract stream 15 and raffinate stream 20 from the illustratedschemes contain desorbent in concentrations relative to the respectiveproduct from the process of between 0% and 100%. The desorbent generallyis separated from raffinate and extract components by conventionalfractionation in, respectively, raffinate column 150 and extract column175 as illustrated in FIG. 1 and recycled to a stream 10′ by raffinatecolumn bottoms pump 160 and extract column bottoms pump 185 to bereturned to the process. FIG. 1 shows the desorbent as bottoms from therespective column, implying that the desorbent is heavier than theextract or raffinate; different commercial units for the separation ofC₈ aromatics employ either light or heavy desorbents, and thus in someapplications the desorbent may be separated at a different locationalong the fractionation columns 150 and 175. The raffinate product 170and extract product 195 from the process are recovered from theraffinate stream and the extract stream in the respective columns 150and 175; the extract product 195 from the separation of C₈ aromaticsusually comprises principally one or both of para-xylene andmeta-xylene, with the raffinate product 170 being principallynon-adsorbed C₈ aromatics and ethylbenzene.

The liquid streams, e.g., the streams of feed 5, desorbent 10, raffinate20, and extract 15 entering and leaving the adsorbent chambers 100 and105 via the active liquid access points or ports 25 effectively dividethe adsorbent chamber 100 and 105 into separate zones which move as thestreams are shifted along the ports 25. It should be noted that whilemuch of the discussion herein refers to FIG. 1 and the location of thestreams in FIG. 1, FIG. 1 illustrates only a current location of thestreams at a single step or a snapshot of the process as the streamstypically shift downstream at different steps of a cycle. As the streamsshift downstream, the fluid composition and the corresponding zonesshift downstream therewith. In one approach, the position of the streamswith regard to the access points or ports 25 of the adsorptiveseparation chambers 100 and 105 remain generally constant with regard toone another as they synchronously progress downstream along the ports25. In one example, the streams each progress a single port 25downstream for each step and each stream occupies each port 25 one timeduring an entire cycle. According to one example, the streams arestepped simultaneously to subsequent ports 25 by rotating a rotary valve300, and are maintained at a particular port 25 or step for apredetermined step-time interval. In one approach, there are betweenabout 4 and 100 ports 25, between about 12 and 48 ports in anotherapproach, and between about 20 and 30 ports in yet another approach, andan equal number of corresponding transfer lines. In one example, theadsorptive separation chamber or chambers 100 and 105 include about 24ports and each stream is shifted to each of the 24 ports 25 during acomplete cycle so that each stream occupies each port 25 andcorresponding transfer line during the cycle. In this example, a cyclemay be between about 20 and about 40 minutes in one approach and betweenabout 22 and 35 minutes in another approach. In one approach, astep-time interval is between about 30 seconds and about two minutes. Inanother approach, the step-time interval is between about 45 seconds andabout one minute thirty seconds. In yet another approach, the step-timeinterval is between about 50 seconds and about one minute and 15seconds. An example of a typical step-time interval may be about 1minute.

With this in mind, FIG. 8 illustrates a snapshot of the compositionalprofile of the fluid within an adsorptive separation chamber (a singleadsorptive separation chamber 100 is illustrated in FIG. 8 forsimplicity) and the corresponding zones into which the adsorptiveseparation chamber 100 is divided. The adsorption zone 50 is locatedbetween the feed inlet stream 5 and the raffinate outlet stream 20. Inthis zone, the feed stream 5 contacts the adsorbent, an extractcomponent is adsorbed, and a raffinate stream 20 is withdrawn. Asillustrated in the figure, the raffinate stream 20 may be withdrawn at alocation where the composition includes raffinate fluid 454 and littleif any extract fluid 450. Immediately upstream with respect to fluidflow is the purification zone 55, defined as the adsorbent between theextract outlet stream 15 and the feed inlet stream 5. In thepurification zone 55, the raffinate component is displaced from thenonselective void volume of the adsorbent and desorbed from the porevolume or surface of adsorbent shifting into this zone by passing aportion of extract stream material leaving the desorption zone 60. Thedesorption zone 60, upstream of the purification zone 55, is defined asthe adsorbent between the desorbent stream 10 and the extract stream 15.The desorbent passing into this zone displaces the extract componentwhich was adsorbed by previous contact with feed in the adsorption zone50. The extract stream 15 may be withdrawn at a location of the chamber100 that includes extract fluid 450 and little if any raffinate fluid454. A buffer zone 65 between the raffinate outlet stream 20 and thedesorbent inlet stream 10 prevents contamination of the extract, in thata portion of the desorbent stream enters the buffer zone to displaceraffinate material present in that zone back into the adsorption zone50. The buffer zone 65 contains enough adsorbent to prevent raffinatecomponents from passing into the desorption zone 60 and contaminatingthe extract stream 15.

Each of the zones described above generally are effected throughmultiple compartments or “beds” as described in U.S. Pat. No. 2,985,589.The positions of the various streams described are structurallyseparated from one another by a horizontal liquidcollection/distribution grid. Each grid is connected to a transfer linedefining a transfer point at which process streams enter and leave theadsorbent chamber. This arrangement facilitates the distribution offluids within the chamber through eliminating channeling and otherinefficiencies, prevents convective back-mixing of fluid in a directionopposite to that of primary fluid flow, and prevents migration ofadsorbent through the chamber. Each of the zones described above usuallycomprises a plurality of 2 to 10, and more usually 3 to 8, beds. Atypical simulated-moving-bed adsorption unit comprises 24 beds ofadsorbent.

It is readily apparent in FIG. 1 that when a transfer line at an accesspoint 25 which is being used to transport a particular stream into orout of the adsorbent chamber is left idle at the end of a step it willremain full of the compounds forming that stream until these compoundsare removed from the line by a second flowing stream. In this regard, itshould be noted that only active transfer lines, i.e. those linespresently facilitating flow of fluid therethrough, are illustrated inFIG. 1, although intermediate transfer lines are present at each of theports 25 along the chambers 100 and 105 to facilitate fluid flow uponshifting of the fluid streams to subsequent ports 25. The residual fluidor compounds left in the now unused transfer line after a stream shiftsto a subsequent transfer line, will therefore be either withdrawn fromthe process as the initial part of a process stream removed from theprocess or forced into the adsorbent chamber when the transfer linecarries a stream into the adsorbent chamber. FIG. 13 illustrates aprevious system showing unused transfer lines as dashed lines andtransfer lines currently occupied by a stream, e.g. stream 920 as solidlines extending from ports of the adsorptive separation chamber 902.

Returning to FIG. 1, as described above, the presence of residual fluidin the transfer lines can have deleterious effects on the performance ofa simulated-moving-bed adsorptive separation process. For example,residual raffinate in a transfer line which previously had been used toremove the raffinate stream 20 from the adsorbent chamber may be flushedinto the adsorbent chamber 105 with the feed stream 5 when it shifts tothat transfer line in a subsequent step. Similarly, residual feed in atransfer line which previously had been used to introduce the feedstream 5 to the adsorbent chamber may be removed from the transfer linewith the extract stream 15 when it shifts to that transfer line in asubsequent step. Likewise, residual extract in a transfer line whichpreviously had been used to remove the extract stream from the adsorbentchamber may be flushed back into the adsorbent chamber 100 with thedesorbent stream 10 when it subsequently arrives at that transfer line.

In accordance with one aspect, a primary flush of the process and systemincludes a primary flush in 30, which flushes residual feed within thetransfer line previously occupied by the feed stream 5 into theadsorptive separation chamber 105, and more particularly into thepurification zone 55. The primary flush in 30 may advantageously bedirected to the transfer line of the purification zone 55 near thetransfer line currently occupied by the feed stream 5 to introduce theresidual feed into the adsorptive separation chamber 105 near the feedstream 5 so that the residual feed can be separated therein. In oneexample, the primary flush in 30 may be directed to a transfer line ofthe purification zone 55 within two transfer lines of the feed stream 5,and more preferably to a transfer line adjacent to the feed stream 5, asillustrated in FIG. 1. In one approach, the primary flush in 30 utilizesflush fluid including primarily the preferentially adsorbed component,desorbent, and/or inert components. In other words, the flush fluidpreferably includes little if any of the non-preferentially adsorbedcomponent of the feed, to restrict contamination of the extract stream15 when the extract stream arrives at the transfer line during asubsequent step.

The primary flush of the process and system may include a primary flushout 35 to flush the residual extract fluid from the transfer linepreviously occupied by the extract stream away from the adsorbentchamber. The extract fluid along with the primary flush flushing fluidis then transferred to the primary flush in 30 transfer line as theflush fluid and is utilized to flush the residual feed from the transferline previously occupied by the feed stream into the purification zoneof the adsorptive separation chamber 105 as described previously. In oneapproach, the primary flush out 35 utilizes fluid from the desorptionzone 60 of the chamber 100 to flush the transfer line that includesprimarily desorbent. In this manner, after the primary flush out 35flushes the residual extract fluid within the transfer line previouslyoccupied by the extract stream 15, very little extract fluid remains inthe transfer line. Advantageously, by coupling the primary flush out 35with the primary flush in 30, residual fluid in the transfer lines canbe used for flushing other transfer lines, reducing the overall amountof fluid required by the process and increasing the yield of the processby capturing these fluids, while achieving the transfer line flushingpurposes discussed previously. In addition, the pairing of the primaryflushes provides the flush fluid for the primary flush in 30, whichincludes primarily desorbent and the preferentially adsorbed componentfrom the residual extract fluid. Likewise, this provides a flush fluidfor the primary flush in 30 that includes very little of thenon-preferentially adsorbed component. In one example, the flush fluidfor the primary flush in 30 includes more than about 99 wt. % desorbentand the preferentially adsorbed component. In another example, the flushfluid includes less than about 0.005 wt. % of the non-preferentiallyadsorbed component(s).

According to one approach, a secondary flush 40 is used to flushresidual fluid from a transfer line that will subsequently be occupiedby the extract stream 15 to remove contaminants from the transfer line.The secondary flush 40 advantageously provides increased purity of theextract stream by removing contaminants from the transfer line beforethe transfer line is used to withdraw the extract stream 15therethrough. Previous systems utilized a flush of desorbent into thetransfer line and toward the adsorptive separation chamber to flush thecontents of the transfer line that will subsequently be used forwithdrawal of the extract stream. This flush is sent through thetransfer line toward the adsorptive separation chamber and into thepurification zone of the adsorptive separation chamber to providepurification thereof.

It has been identified that the secondary flush of previously systemsdiscussed previously created a utilities or energy penalty.Specifically, because the secondary flush 40 uses desorbent to flush theresidual preferentially adsorbed component/desorbent fluid in thetransfer line into the adsorptive separation chamber, this transfer lineincludes almost exclusively desorbent after the secondary flush. Theresidual desorbent within this transfer line is subsequently withdrawnas an initial surge of fluid by the extract stream prior to the removalof extract. The extract stream, including this surge of residualdesorbent, is directed to the extract fractionation column 175, where itis fractionated out as a bottoms product and recycled with desorbentrecycle stream to the first chamber 100. However, in order to enter thecolumn 175, the surge of residual desorbent within the transfer line atthe beginning of the removal of the extract must also be heated prior toentering the extract column 175 for fractionation. For example, whenpara-xylene is being separated from a feed stream of mixed xylenes, thedesorbent withdrawn with the extract stream is heated from about 150° C.to about 300° C., resulting in an energy or utilities penalty. In otherwords, because this initial slug of desorbent contains very little ifany of the desired extract product, it requires a substantial energyinput to increase the temperature to the extract fractionation columnbottoms outlet temperature while not providing a benefit in terms ofincreased extract product yield.

In order to avoid this utilities and energy penalty, according to oneaspect a secondary flush 40 flushes residual fluid from the transferline 45 away from the adsorptive separation chamber 100, the opposite ofprevious systems, so that residual desorbent does not build up withinthe transfer line 45. It should be noted that the transfer line 45 isused for the secondary flush 40 in the step illustrated in FIG. 1,however, during previous or subsequent steps the secondary flush 40 mayshift along with the streams and be used to remove residual fluid fromother transfer lines. More specifically, rather than using a desorbentstream to flush the residual fluid from the transfer line 45, which mayinclude primarily the preferentially adsorbed component and desorbentremaining in the transfer line after the primary flush in 30, fluid fromthe purification zone, adjacent to the transfer line port 45′corresponding to the transfer line is used to flush the residual fluidaway from the adsorbent chamber 100. The secondary flush stream may thenbe transferred for further processing. In one approach, the secondaryflush is sent by a line 40′ to a fluid recycle line 10′. The fluidrecycle line 10′ may include primarily desorbent that is separated viathe fractionation columns 150 and 175 and recycled back to theadsorptive separation chamber 100 where it is reused in the process. Inone approach, the secondary flush stream is sent via line 40′ to abottoms portion 155 of the raffinate fractionation column 150 where itis combined with the desorbent separated by the raffinate fractionationcolumn 150 and sent to the fluid recycle line 10′ via a raffinatebottoms pump 160. In another approach the secondary flush stream is sentvia line 40′ to a bottoms portion 180 of the extract fractionationcolumn 175 where it is combined with the desorbent separated by theextract fractionation column 175 and sent to the fluid recycle line 10′via a extract bottoms pump 185.

Because this fluid from the purification zone 55 is similar incomposition to the extract stream 15 that will be subsequently withdrawnfrom the transfer line 45, the residual fluid remaining in the bed lineafter the modified secondary flush 40 will advantageously be similar incomposition to the desired extract composition. To this end, in oneexample the transfer line 45 is flushed by the secondary flush 40 withintwo transfer lines or ports from the transfer line currently occupied bythe extract line 15, and more preferably within one transfer line orport from the transfer line currently occupied by the extract line 15,since purification zone fluid adjacent to ports near the extracttransfer line will have compositions most similar to the extract stream15. In one example, the purification zone fluid has more than about 99%desorbent and preferentially adsorbed component. In another example, thepurification zone fluid has less than about 0.005% of thenon-preferentially adsorbed component(s). Further, when a primary flushin 30 is used to flush residual feed as described previously, thesecondary flush 40 according to one approach is positioned between thetransfer line currently occupied by the extract stream 15 and thetransfer line currently occupied by the primary flush in 30 so that thetransfer line 45 is primarily filled with residual fluid from theprimary flush in 30 rather than the feed stream 5. This approachadvantageously reduces the extent of contamination of the extract stream15 with residual feed.

Further, in one approach, fluid within the transfer line 45 that willsubsequently be withdrawn with the extract stream 15 will be sent to theextract fractionation column 175 to be separated via a distillation. Theresidual fluid within the transfer line 45 that is sent with the extractstream to the extract fractionation column 175 is heated within extractfractionation column 175. Because this residual fluid is similar incomposition to the extract stream 15 fractionation of this fluid willresult in increased recovery of the desired extract product 195. Thus,unlike prior systems, fluid remaining in the transfer line 45 from thesecondary flush 40 that is subsequently taken up with the extract stream15 and sent to the extract fractionation column 175 will not result inan unnecessary utilities penalty, because distillation of this fluidwill result in additional yield of the desired extract product 195rather than primarily desorbent.

In accordance with another aspect illustrated in FIG. 2, the extractstream 15 may be withdrawn through a transfer line during a step aspreviously described. In this approach, the extract stream 15 iswithdrawn along with residual fluid remaining in the transfer line sothat the extract stream flushes the residual fluid away from thetransfer line. An initial slug of the extract stream, including at leasta portion of the residual fluid, is directed through the transfer lineto a first destination. A subsequent portion of the extract stream isthen directed through the transfer line to a second destination. Atleast a portion of the residual fluid within the transfer line isdirected to the first destination. In one example at least about 90% ofthe residual fluid is directed to the first destination. In anotherexample, at least about 95% of the residual fluid is directed to thefirst destination. In one approach, the second destination is an inlet190 of the extract fractionation column 175. The first destination maybe a recycle line 10′ for recycling the extract stream and the portionof the residual fluid to the adsorptive separation chamber 100.

As illustrated in FIG. 2, a primary flush in 30 may be utilized to flushresidual feed fluid remaining in the transfer line previously occupiedby the feed stream 5 into the adsorptive separation chamber 105 asdescribed previously to restrict the residual feed fluid from beingwithdrawn with the extract stream as the residual fluid in the transferline when the extract stream 15 arrives at the transfer line in asubsequent step. The flushing fluid preferably includes primarilydesorbent and/or the preferentially adsorbed component and includes verylittle of the non-preferentially adsorbed component so that the residualfluid remaining in the transfer line after the primary flush in 30includes very little of the non-preferentially adsorbed component. Inone approach, the flushing fluid includes less than about 1% of thenon-preferentially adsorbed component and in another example includesless than about 0.005% of the non-preferentially adsorbed component. Asdescribed previously, residual extract remaining in a transfer linepreviously occupied by the extract stream 15 may be flushed from thetransfer line via a primary flush out 35, and the residual extract fluidmay be transferred to the primary flush in 30 transfer line to be usedas the flushing fluid for the primary flush in 30. The residual extractfluid may be flushed via the primary flush out 35 by withdrawing fluidfrom the desorption zone 60 adjacent to the port 25 in communicationwith the primary flush out 35 transfer line. In this regard, theresidual fluid within the transfer line when the extract stream 15 isshifted thereto, may include primarily residual extract and flushingfluid withdrawn from the desorption zone 60 via the primary flush out35, e.g. residual extract and desorbent.

Turning to more of the particulars in FIG. 2, according to thisapproach, the extract stream 15 is withdrawn through the transfer lineincluding the residual fluid so that an initial slug of the extractstream will include the residual fluid that remained in the transferline prior to the arrival of the extract stream 15. As mentionedpreviously, this initial slug of the extract stream may be sent to arecycle line 10′ to be recycled back to the adsorptive separationchamber 100. To this end, the initial slug of the extract stream may besent to a raffinate fractionation column bottoms portion 155. At theraffinate column bottoms portion 155 the slug of fluid is combined withfluid exiting the bottom of the raffinate fractionation column 150,which in one example includes primarily desorbent that has beenseparated in the raffinate fractionation column 150. A raffinate columnbottoms pump 160 may be used to direct this slug of fluid and thedesorbent back to the adsorptive separation chamber 100 through therecycle line 10′. Alternatively, the initial slug of the extract streammay be sent to an extract fractionation column bottoms portion 180. Atthe extract column bottoms portion 180 the slug of fluid is combinedwith fluid exiting the bottom of the extract fractionation column 175,which in one example includes primarily desorbent that has beenseparated in the extract fractionation column 175. An extract columnbottoms pump 185 may be used to direct this slug of fluid and thedesorbent back to the adsorptive separation chamber 100 through therecycle line 10′.

In this manner, at least a portion of the residual fluid withdrawn withthe extract stream 15 is not directed to the extract fractionationcolumn inlet 190. Because the residual fluid in the transfer line fromthe primary flush 30 will contain a greater percentage of desorbent thanthe extract stream 15, this excess desorbent is advantageously notseparated in the extract fractionation column 175. Because fluidentering the extract fractionation column inlet 190 is heated, if theexcess desorbent in the residual fluid was introduced into the extractfractionation column 175 it would be heated to the bottoms outlettemperature without providing additional yield of the extract product,and thus incurring an energy penalty. Thus, by diverting the initialslug of fluid so that excess desorbent is not introduced into theextract fractionation column 175, the amount of energy required by thesystem is reduced.

According to one aspect, the extract stream 15 is withdrawn from theadsorptive separation chamber 100 and sent along a transfer line 15′. Inone approach a rotary valve 300 is provided so that the extract stream15 is withdrawn through the transfer line and directed to the rotaryvalve where it is combined with a single extract transfer line 15′ asillustrated in FIG. 2, although other configurations are contemplatedherein, including providing a dedicated extract transfer line 15′ foreach transfer line of the adsorptive separation chambers 100 and 105.The transfer line 15′ may have one extract inlet line 205 in fluidcommunication with the extract fractionation column inlet 190. Thetransfer line 15′ may have another bottoms portion line 210 incommunication with one or both of the extract column bottoms portion 180and the raffinate column bottoms portion 155. A valve 215 may beprovided for diverting the flow of the extract stream 15 between theextract column inlet line 205 and the extract column bottoms portionline 210. In this manner, the process includes moving the valve 215 to afirst position to direct the initial portion extract stream 15 includingat least a portion of the residual fluid through the extract columnbottoms portion line 210 to one of the extract column bottoms portion180 and the raffinate column bottoms portion 155. In this example, theprocess includes diverting the valve 215 to a second position to directthe extract stream 15 through the extract column inlet line 205 andtoward the extract fractionation column inlet 190 for separation of theextract stream 15 therein.

In accordance with one aspect, the extract stream, including at least aportion of the residual fluid flushed from the transfer line by theextract stream, is directed to the first destination, e.g. one or bothof the extract column and the raffinate column bottoms portions 180 and155 for a first predetermined time or predetermined portion of astep-time interval (when the extract stream occupies a current transferline). The extract stream is then directed to the second destination,e.g. the inlet of the extract fractionation column 175 for a secondpredetermined time or predetermined portion of the step-time interval.The first predetermined time may be selected based on a flow rate of theextract stream to flush a predetermined amount of the residual fluid inthe transfer line to the second destination or a predetermined amount offluid to the second destination. In one example, the first predeterminedtime may be sufficient to direct a volume of fluid of about 50% to about250% of a volume of the transfer line and associated valving, and inanother example from about 80% to about 150% of the volume of thetransfer line and associated valving, to the first destination. In oneapproach, the second predetermined time may be the remainder of thestep-time interval so that the extract stream 15 is directed to theextract column inlet 190 for the remainder of the step-time interval forseparation of the extract stream 15 in the extract fractionation column175. The predetermined times may also be selected to direct all or atleast a portion of the residual fluid in the transfer line to the firstdestination so that the residual fluid is not introduced into theextract fractionation column to provide energy savings. Similarly, afirst predetermined volume of the extract stream may be directed to thefirst destination and a second predetermined volume of the extractstream may be direct to the second destination. The first predeterminedvolume may be the same as described above for the first predeterminedtime. The second predetermined volume may be the remaining volume of theextract stream withdrawn through the transfer line during the step-timeinterval. In one example, the first predetermined time is between about10% and about 90% of the step-time interval. The second predeterminedtime in this example is between about 10% and about 90% of the step timeinterval. In another example, the first predetermined time is betweenabout 20% and about 40% of the step-time interval. The secondpredetermined time in this other example is between about 60% and about80% of the step time interval.

In another approach, the process includes monitoring the composition ofthe extract stream, including any residual fluid therein to determine aquantity or percentage of a component within the composition. Forexample, the component may be one of the preferentially adsorbedcomponent, a desorbent component, or the non-preferentially adsorbedcomponent. The process according to this approach includes directing theextract stream 15 and any residual fluid to the first destination whenthe composition includes the component at a first predetermined leveland directing the extract stream 15 to the second destination when thecomposition includes the component at a second predetermined level. Forexample, the process may include monitoring the composition of theextract stream 15 to determine the amount of the desorbent present inthe stream. According to this example, the process may include directingthe extract stream to the first destination when the amount of desorbentis above a threshold level and directing the extract stream to thesecond destination when the amount of desorbent is below the thresholdlevel. In this manner, the amount of desorbent sent to the extractfractionation column inlet 190 may be reduced.

Advantageously, according to this approach, the secondary flush 40 ofprevious systems may be omitted. In this manner, the process may be usedwith one less active transfer line. For example, the process may useonly six or seven transfer lines rather than seven or eight transferlines, as was required in previous systems. In one approach, the processmay use a rotary valve 300 with only six or seven tracks, includingtracks for the extract, raffinate, feed, and desorbent streams, and alsothe primary flush out 35, the primary flush in 30, and optionally atertiary flush 46. This approach advantageously allows existingadsorptive separation systems with six and seven track rotary valves tobe retrofitted to utilize the invention according to this approach.

Turning now to FIG. 3, an adsorptive separation system and process inaccordance with another aspect is illustrated. According to this aspect,the raffinate stream 20 may be withdrawn through a transfer line duringa step as previously described. In this approach, the raffinate stream20 is withdrawn along with residual fluid remaining in the raffinatestream transfer line so that the raffinate stream 20 flushes theresidual fluid away from the transfer line. This aspect is similar tothat described above and illustrated in FIG. 2 in that an initial slugof the raffinate stream is directed to a first destination. A subsequentportion of the raffinate stream is then directed to a seconddestination. At least a portion of the residual fluid within thetransfer line is directed to the first destination. In one example atleast about 90% of the residual fluid is directed to the firstdestination. In another example, at least about 95% of the residualfluid is directed to the first destination. In one aspect, the seconddestination is an inlet 165 of the raffinate fractionation column 150.The first destination may be a recycle line 10′ for recycling theraffinate stream and the portion of the residual fluid to the adsorptiveseparation chamber 100. In this regard, by recycling a portion of thefluid back to the adsorptive separation chamber 100 the amount of fluidprocessed by the raffinate fractionation column 150.

As illustrated in FIG. 3, in one approach, the transfer line occupied bythe raffinate stream 20 was previously occupied by the desorbent stream10. In this regard, the transfer line may include primarily residualdesorbent fluid when the raffinate stream arrives at the transfer linein a subsequent step.

Turning to more of the particulars in FIG. 3, according to this aspect,the raffinate stream 20 is withdrawn through the transfer line includingthe residual fluid so that an initial slug of the raffinate stream willinclude the residual fluid that remained in the transfer line prior tothe arrival of the raffinate stream 20. As mentioned previously, thisinitial slug of the raffinate stream may be sent to a recycle line 10′to be recycled back to the adsorptive separation chamber 100. To thisend, similar to the approach described previously with regard to FIG. 2,the initial slug of the raffinate stream 20 may be sent to a raffinatefractionation column bottoms portion 155. At the raffinate columnbottoms portion 155 the slug of fluid is combined with fluid exiting thebottom of the raffinate fractionation column 150, which in one exampleincludes primarily desorbent that has been separated in the raffinatefractionation column 150. A raffinate column bottoms pump 160 may beused to direct this slug of fluid and the desorbent back to theadsorptive separation chamber 100 through the recycle line 10′.Alternatively, the initial slug of the raffinate stream 20 may be sentto an extract fractionation column bottoms portion 180. At the extractcolumn bottoms portion 180 the slug of fluid is combined with fluidexiting the bottom of the extract fractionation column 175, which in oneexample includes primarily desorbent that has been separated in theextract fractionation column 175. Similarly, an extract column bottomspump 185 may be used to direct this slug of fluid and the desorbent backto the adsorptive separation chamber 100 through the recycle line 10′.

In this manner, at least a portion of the residual fluid withdrawn withthe raffinate stream 20 is not directed to the raffinate fractionationcolumn inlet 165. Because the residual fluid in the transfer line willcontain a greater percentage of desorbent than the raffinate streamfluid, this excess desorbent is advantageously not sent to and separatedin the raffinate fractionation column 150. Because fluid entering theraffinate fractionation column inlet 165 is heated in the column, if theexcess desorbent in the residual fluid was introduced into the raffinatefractionation column 150 it would be heated without providing additionalyield of the extract product, and thus incurring an energy penalty.Thus, by diverting the initial slug of fluid so that excess desorbent isnot introduced into the raffinate fractionation column 150, the amountof energy required by the system is reduced.

In one approach, the raffinate stream 20 is withdrawn from theadsorptive separation chamber 100 and sent along a transfer line 20′. Inone approach a rotary valve 300 is provided so that the raffinate stream20 is withdrawn through the transfer line and directed to the rotaryvalve 300 where it is combined with a single raffinate transfer line 20′as illustrated in FIG. 3, although other configurations are contemplatedherein, including providing a dedicated raffinate transfer line 20′ foreach transfer line of the adsorptive separation chambers 100 and 105.The transfer line 20′ may have one raffinate inlet line 305 in fluidcommunication with the raffinate fractionation column inlet 165. Thetransfer line 20′ may have another bottoms portion line 310 in fluidcommunication with one or both of the extract column bottoms portion 180and the raffinate column bottoms portion 155. A valve 315 may beprovided for diverting the flow of the raffinate stream 20 between theraffinate column inlet line 305 and the raffinate column bottoms portionline 310. In this manner, the process includes moving the valve 315 to afirst position to direct the initial portion raffinate stream 20including at least a portion of the residual fluid through the raffinatecolumn bottoms portion line 310 to one of the extract column bottomsportion 180 and the raffinate column bottoms portion 155. In thisexample, the process includes moving the valve 315 to a second positionto direct the raffinate stream 20 through the raffinate column inletline 305 and toward the raffinate fractionation column inlet 165 forseparation of the raffinate stream 20 therein.

In one aspect, the raffinate stream 20, including at least a portion ofthe residual fluid flushed from the transfer line by the raffinatestream, is directed to the first destination, e.g. one or both of theextract column and the raffinate column bottoms portions 180 and 155 fora first predetermined time or predetermined portion of a step-timeinterval (when the raffinate stream occupies a current transfer line).The raffinate stream is then directed to the second destination, e.g.the raffinate fractionation column inlet 165 for a second predeterminedtime or predetermined portion of the step-time interval. The firstpredetermined time may be selected based on a flow rate of the raffinatestream 20 to flush a predetermined amount of the residual fluid in thetransfer line to the second destination or a predetermined amount ofoverall fluid to the second destination. In one example, the firstpredetermined time may be sufficient to direct a volume of fluid ofabout 50% to about 250% of a volume of the transfer line and associatedvalving, and in another example from 80% to about 150% of the volume ofthe transfer line and associated valving, to the first destination. Inone approach, the second predetermined time may be the remainder of thestep-time interval so that the raffinate stream 20 is directed to theraffinate column inlet 165 for the remainder of the step-time intervalfor separation of the raffinate stream 20 in the raffinate fractionationcolumn 150. The predetermined times may also be selected as other valuesin order to direct all or at least a portion of the residual fluid inthe transfer line to the first destination so that the residual fluid isnot introduced into the raffinate fractionation column 150 to provideenergy savings. In one example, the first predetermined time is betweenabout 10% and about 90% of the step-time interval. The secondpredetermined time in this example is between about 10% and about 90% ofthe step time interval. In one example, the first predetermined time isbetween about 10% and about 30% of the step-time interval. The secondpredetermined time in this example is between about 70% and about 90% ofthe step time interval. Similarly, a first predetermined volume of theraffinate stream may be directed to the first destination and a secondpredetermined volume of the raffinate stream may be direct to the seconddestination. The first predetermined volume may be the same percentageof the volume of the transfer line and associated valving as describedabove for the first predetermined time. The second predetermined volumemay be the remaining volume of the raffinate stream withdrawn throughthe transfer line during the step-time interval.

In another aspect, the process includes monitoring the composition ofthe raffinate stream 20, including any residual fluid therein todetermine a quantity or percentage of a component within thecomposition. For example, the component may be one of the preferentiallyadsorbed component, a desorbent component, or the non-preferentiallyadsorbed component. The process according to this approach includesdirecting the raffinate stream 20 and any residual fluid to the firstdestination when the composition includes the component at a firstpredetermined level and directing the raffinate stream 20 to the seconddestination when the composition includes the component at a secondpredetermined level. For example, the process may include monitoring thecomposition of the raffinate stream to determine the amount of thedesorbent present in the stream. According to this example, the processmay include directing the raffinate stream to the first destination whenthe amount of desorbent is above a threshold level and directing theraffinate stream to the second destination when the amount of desorbentis below the threshold level. In this manner, the amount of desorbentsent to the raffinate fractionation column inlet 165 may be reduced.

Turning to FIG. 4, according to another aspect an adsorptive separationprocess includes a primary flush out 405 for flushing residual fluid inan intermediate transfer line of the purification zone 55, between thetransfer line occupied by the feed stream 5 and the transfer lineoccupied by the extract stream 15 away from the adsorptive separationchamber 100 and 105 to remove at least a portion of the residual fluidfrom the intermediate transfer line. The process according to thisaspect further includes directing the residual fluid flushed from theintermediate transfer line to another transfer line that is not atransfer line of the purification zone 55 to restrict the residual fluidfrom being introduced into the purification zone 55. In this manner, theresidual fluid in the intermediate transfer line is not injected backinto the purification zone as with previous systems, where components ofthe residual fluid would be separated, but without the benefit offlowing through the entire purification zone 55 prior to withdrawal viathe extract stream 15 at the top of the purification zone 55.

In one aspect, the residual fluid flushed by the primary flush out 405is transferred to and combined with the feed stream 5 to be introducedinto the adsorptive separation chamber 105 with the feed stream 5 viathe feed stream transfer line. In this manner, components of theresidual fluid introduced with the feed stream may be separated withinadsorptive separation unit with the feed fluid introduced via the feedstream 5. This provides more complete component separation than if theresidual fluid were introduced directly into the purification zone 55through an intermediate transfer line, because components in theresidual fluid may flow through the entire purification zone 55 betweenthe feed stream 5 and the extract stream 15 prior to being withdrawn viathe extract stream 15. This approach may increase the purity of theextract stream 15 due to the more complete separation of the componentsof the residual fluid.

The residual fluid remaining in the intermediate transfer line that isflushed via the primary flush out 405 according to one approach mayinclude residual feed fluid. To this end, the intermediate transfer linemay have previously been occupied by the feed stream 5, so that theintermediate transfer line includes the residual feed fluid when thefeed stream is shifted away therefrom at the end of a step. The residualfeed fluid may advantageously be combined with the feed stream 5 andinjected into the purification zone via the feed stream transfer lineand port so components in the residual feed fluid are separated to aboutthe same extent as the components of the feed stream 5 itself.

Because the pressure in the primary flush out 405 transfer line may belower than the pressure in the feed stream transfer line, the primaryflush fluid may need to be pumped in order to overcome the pressuredifferential and be combined with the feed stream 5. In this regard, apump 410 may be provided for pumping the primary flush fluid through theintermediate transfer line and combining it with the feed stream 405. Inone approach, the system may include a rotary valve, with the primaryflush being flushed through the intermediate transfer line and to therotary valve 300 where it is combined with the feed stream 5. However,at certain transfer lines or ports 25 along the adsorptive separationchambers 100 and 105 where two or more adsorptive separation chamber 100and 105 are used, the pressure at the feed stream 5 may be higher thanthe pressure of the primary flush out stream 405 where the primary flushout stream 405 is transferred between a transfer line near the bottom ofthe adsorptive separation chambers 100 and 105 to join the feed stream 5near the top of the other one of the adsorptive separation chambers 100and 105. In these positions, residual feed in the line may surge intothe extract stream because adjacent transfer lines are often in fluidcommunication with each other in processes utilizing a rotary valve 300.Thus, in one approach the pump 410 is positioned downstream of therotary valve as illustrated in FIG. 4 to restrict the residual feed inthe intermediate transfer line from back-flushing into the extractstream 15 when the streams are located at certain positions along theadsorptive separation chambers 100 and 105.

According to one aspect, the primary flush out 405 includes withdrawingfluid from the purification zone 55 of the adsorptive separation chamber100 through a port 25 of the transfer line 415. The purification zonefluid is withdrawn from a location in the purification zone 55 adjacentto the port 25 and transferred into the intermediate transfer line inorder to flush the residual fluid in the intermediate transfer line awayfrom the adsorptive separation chamber 100. Flushing the intermediatetransfer line 415 with purification zone fluid advantageously fills thetransfer line 415 with fluid that is higher in concentration of thepreferentially adsorbed component than the non-preferentially adsorbedcomponent to reduce contamination of the extract stream 15 when theextract stream 15 arrives at the intermediate transfer line 415 in asubsequent step. In one approach, the purification zone material iswithdrawn into the transfer line at a location near the transfer linecurrently occupied by the extract stream 15 so that the fluid within thepurification zone 55 that is being withdrawn is similar in compositionto the extract stream fluid. In one approach, the purification zonefluid is withdrawn through a port 25 and into a transfer line within twotransfer lines from the transfer line currently occupied by the extractstream 15. In another approach, the purification zone fluid is withdrawnthrough a port 25 and into an intermediate transfer line of thepurification zone 55 adjacent to the transfer line currently occupied bythe extract stream 15. In this manner, the composition of thepurification zone fluid used to flush the intermediate transfer linethat will remain in the transfer line after the primary flush out willbe similar in composition to the extract stream fluid and include only asmall amount if any of the non-preferentially adsorbed components fromthe feed stream which would otherwise contaminate the extract stream 15when it arrives at the intermediate transfer line during a subsequentstep. In one example, the purification zone fluid withdrawn from theadsorptive separation chamber includes less than about 0.5% of thenon-preferentially adsorbed component. In another example, thepurification zone material used for the primary flush out 405 includesless than about 0.005% of the non-preferentially adsorbed component. Aswill be readily understood, according to this aspect, by transferringthe primary flush out 405 and combining it with the feed stream 5, oneless transfer line may be required when compared to a system thattransfers the residual fluid from the primary flush out to anotherintermediate transfer line.

A process and system for adsorptive separation of components from a feedstream according to another aspect is illustrated in FIG. 5. The processaccording to this aspect may include a primary flush out 505 similar tothat described above in regard to FIG. 4. In contrast to the primaryflush out 405 described above, however, the primary flush out 505according to this aspect is directed to another transfer line of thepurification zone 55 rather than combined with the feed stream 5. Moreparticularly, the process includes flushing residual fluid within anintermediate transfer line 510 of the purification zone 55 between thefeed stream 5 transfer line and the extract stream 15 transfer line awayfrom the adsorptive separation chamber 100 or 105 to remove at least aportion of the residual fluid from the intermediate transfer line 510via a primary flush out 505. The process further includes directing theresidual fluid flushed from the intermediate transfer line 510 toanother intermediate transfer line 515 of the purification zone 55 toflush residual fluid in the other intermediate transfer line 515 intothe purification zone adjacent to the other intermediate transfer line515 via a primary flush in 520.

According to one aspect, the other intermediate transfer line 515includes residual feed fluid remaining in the intermediate transfer line515 from the feed line 5 that occupied the intermediate transfer line515 during a previous step. Thus, when flushing fluid is introduced intothe intermediate transfer line 515 during the primary flush in 520, theresidual feed fluid is introduced into the purification zone 55 of theadsorptive separation chamber 100 or 105. However, because the feedstream has already been shifted downstream of the primary flush intransfer line 515, the residual feed is introduced in an intermediatelocation of the purification zone. Thus, in one approach, in order toincrease the amount of separation of components that occurs in theresidual feed material in the purification zone 55, the primary flush intransfer line 515 is positioned between the primary flush out transferline 510 and the transfer line currently occupied by the feed stream 5,so that the residual feed fluid is introduced into a portion of thepurification zone near the feed stream. In one example, the primaryflush in transfer line 515 is positioned within two transfer lines ofthe feed stream transfer line and in another example within one transferline of the feed stream transfer line to increase the amount ofseparation of the components of the residual feed fluid that occurs inthe purification zone 55.

The description above regarding the primary flush out 405 in regard toFIG. 4 also applies to the primary flush out 505 according to the aspectillustrated in FIG. 5 except that because the residual fluid in theintermediate transfer line is transferred to the transfer line 515 forthe primary flush in 520, the intermediate transfer line 510 will notinclude primarily feed fluid when the primary flush out begins as wasthe case with primary flush out 405 described above. In this regard,residual fluid within the intermediate transfer line 510 will insteadinclude fluid previously flushed from the primary flush out transferline 510 to the primary flush in transfer line 515 during a previousstep and thus will include primarily purification zone fluid withdrawnfrom the purification zone 55 as described above with regard to primaryflush out 405.

Turning to FIG. 6, a process for adsorptive separation of components ofa feed stream in accordance with another aspect is shown. According tothis aspect, as described previously, an extract stream 15 is withdrawnfrom the adsorptive separation chamber 100. The extract stream 15 may betransferred to an extract separation device, e.g. the extractionfractionation column 175 for separation of the preferentially adsorbedcomponent from the extract stream 15. The extract stream 15 may bedirected to the extract fractionation column inlet 190 via an extractstream removal line 15′.

The process according to this aspect includes flushing an intermediatetransfer line 610 of a desorption zone 60 between the extract stream 15transfer line and the desorbent stream 10 transfer line away from theadsorptive separation chamber 100 via a secondary flush 605 to removeresidual fluid from the intermediate transfer line 610. The processfurther includes directing the residual fluid flushed from theintermediate transfer line 610 to a downstream separation apparatus toseparate components of the residual fluid. According to one aspect,since the intermediate transfer line 610 was previously occupied by theextract stream 15, the residual fluid in the intermediate transfer line610 includes primarily extract fluid when the secondary flush 605begins. In this regard, the residual extract fluid can be directed tothe downstream separation apparatus to separate the preferentiallyadsorbed component from the extract fluid to increase the yield of thepreferentially adsorbed component.

According to one aspect, the residual extract fluid flushed from theintermediate transfer line 610 is directed to the extract fractionationcolumn inlet 175 so that the preferentially adsorbed component can beseparated from the residual extract fluid via distillation to increasethe yield of the extract product 195.

By one aspect, the secondary flush 605 includes flushing the residualfluid in the intermediate transfer line 610 with desorption zoneflushing fluid withdrawn from the desorption zone 60 of the adsorptiveseparation chamber 100 via a corresponding port of the intermediatetransfer line 610. In one example, intermediate transfer line 610 iswithin two transfer lines of the transfer line currently occupied by thedesorbent stream 10 and in another example is within one transfer lineof the transfer line currently occupied by the desorbent stream 10 sothat the desorption zone flushing fluid is similar in composition to thedesorbent stream 10. In this manner, the desorption zone flushing fluidremains in the intermediate transfer line 610 after the secondary flush605 has occurred. Upon shifting of the desorbent stream to theintermediate transfer line 610 in a subsequent step the residualdesorption zone fluid remaining in the intermediate transfer line 610 isintroduced into the adsorptive separation chamber 100 with the desorbentstream so that the desorbent zone fluid is similar in composition to thedesorbent stream 10.

In accordance with another aspect, a process is provided for adsorptiveseparation of components of a feed stream that includes flushing anintermediate transfer line located between two of the feed stream 5,extract stream 15, desorbent stream 10, and raffinate stream 20 toremove residual fluid from the intermediate transfer line. The process,according to this aspect includes generally flushing the intermediatetransfer line at a dynamic or non-constant volumetric flow rate duringat least two different portions of a step-time interval.

As described previously, in accordance with various aspects of theinvention, countercurrent adsorptive separation includes introducing afeed stream 5, comprising at least one preferentially adsorbed componentand at least one non-preferentially adsorbed component, and a desorbentstream 10 into two different ports 25 via two different correspondingtransfer lines along a multi-bed adsorptive separation chamber having aplurality of beds that are serially connected in fluid communication andcomprising a predetermined number of spaced ports with correspondingtransfer lines in fluid communication therewith for introducing andremoving fluid into and from the adsorptive separation chamber andwithdrawing an extract stream 15 and raffinate stream 20 through twodifferent ports of the multi-bed adsorptive separation chamber via twodifferent corresponding transfer lines. The various streams that areintroduced and withdrawn to and from the adsorptive separation chamber100 and 105 are sequentially shifted or stepped downstream to subsequentports. The various streams are typically stepped simultaneously tosubsequent ports 25, for example by rotating a rotary valve 300, and aremaintained at a particular port 25 or step for a predetermined step-timeinterval. As discussed above, in one approach, there are between about 4and 100 ports 25, between about 12 and 48 ports in another approach, andbetween about 20 and 30 ports in yet another approach, and an equalnumber of corresponding transfer lines. In one example, the adsorptiveseparation chamber or chambers 100 and 105 include about 24 ports andeach stream is shifted to each of the 24 ports 25 during a completecycle so that each stream occupies each port 25 and correspondingtransfer line during the cycle. In this example, a cycle may be betweenabout 20 and about 40 minutes in one approach and between about 22 and35 minutes in another approach. In one approach, a step-time interval isbetween about 30 seconds and about two minutes. In another approach, thestep-time interval is between about 45 seconds and about one minutethirty seconds. In yet another approach, the step-time interval isbetween about 50 seconds and about one minute and 15 seconds.

In this regard, the process includes flushing an intermediate transferline between two lines currently occupied by two of the typical streams,including the feed stream 5, the desorbent stream 10, the extract stream15, and the raffinate stream 20 at a non-uniform or dynamic volumetricflow rate during the step-time interval. According to one aspect theprocess includes flushing the intermediate transfer line at a first flowrate for a first portion of the step-time interval. The process includesflushing the intermediate transfer line at a second flow rate for asecond portion of the step-time interval later during the step-timeinterval than the first portion. In this manner, a greater volume offluid is flushed from the intermediate transfer line during one of thefirst and second portion of the step-time interval than during the otherportion. Flushing the transfer line at a non-constant flow rate mayprovide performance advantages in terms of the composition of fluidflushed into or from the intermediate transfer line as well as thetiming of introducing fluids to or from the intermediate transfer line.

In one aspect, the non-constant flow rate may include a ramped orexponentially increasing or decreasing flow rate that increases ordecreases during at least a portion of the step-time interval. In thisregard, the ramped flow rate may increase or decrease during a portionof the step-time interval and may vary linearly or non-linearly, e.g.exponentially during that time. By another aspect, the non-constant flowrate may include step increases or decreases in the flow rate so thatone or both of the first flow rate and the second flow rate is constantand one is different than the other of the first flow rate and thesecond flow rate. In yet another aspect, the non-constant flow rate mayinclude a combination of ramped portions and step increases anddecreases in the volumetric flow rate. The non-constant flow rate mayalso include additional flow rates during additional portions of thestep-time interval. The flow rate may increase, decrease, or remainunchanged during any particular step. In addition the flow rate may bechanged from the initial value to a higher value, lower value or zero atthe conclusion of a step. FIGS. 10-12 illustrate examples ofnon-constant flow rates in accordance with various aspects of theinvention. FIG. 10 illustrates a ramped flow rate 1015 that increasesover time 1020 during at least a portion of the step-time interval. Inthis example, a first flow rate 1005 is lower than a second flow rate1010 so that a greater volume of fluid is flushed during the secondportion of the step-time interval than during the first portion. Inanother example, the ramped flow rate decreases over time so that afirst flow rate is higher than a second flow rate so that a greatervolume of fluid is flushed during the first portion of the step-timeinterval than during the second portion. FIG. 11 on the other handillustrates an example of a non-constant stepped flow rate. In thisexample, the flow rate 1115 is at a first generally constant flow rate1105 during a first portion of the step-time interval 1120 and increasesto a second and generally constant higher flow rate 1110 during thesecond portion of the step-time interval 1120. In another example, thestepped flow rate has a second generally constant flow rate during thesecond portion of the step-time interval that is lower than a first flowrate so that so that more volume of fluid is flushed during the firstportion of the step-time interval. The volumetric flow rate during oneof the first and second portions may be zero according to variousaspects. In yet another example, illustrated in FIG. 12, the flow rate1215 at a first portion of the step-time interval 1220 begins at a firstflow rate 1205 and then includes second flow rate 1210 thatexponentially decreases over time during a second portion of thestep-time interval 1220. Other flow rate profiles are also contemplatedin accordance with various aspects of the invention that have differentfirst and second flow rates during corresponding first and secondportions of the step-time interval and may there may be additionalportions of the step-time interval with still other flow rates.

In accordance with one aspect, one of the first and second flow rates issufficient to flush between about 50% and 400% of the volume of thetransfer line being flushed and associated valving so that most or allof the residual fluid within the transfer line is flushed during thefirst or second portion of the step-time interval. According to anotheraspect, one of the first and second flow rates is sufficient to flushbetween about 75% and about 200% of the transfer line and associatedvalving volume during the first or second portion of the step-timeinterval. In yet another aspect, one of the first and second flow ratesis sufficient to flush between about 90% and about 150% of the transferline and associated valving volume during the first or second portion ofthe step-time interval. The other of the first and second flow ratesaccording to various aspects may be sufficient to flush between about 0%and about 75% of the transfer line and valving volume in one approach,between about 0% and about 50% of the transfer line and valving volumein another approach, and between about 0% and about 25% of the transferline valving volume in yet another approach.

According to one aspect, the first flow rate is higher than the secondflow rate so that a greater volume of fluid is flushed during the firstportion of the step-time interval than during the second portion of thestep-time interval. The process according to this aspect may beparticularly beneficial when the process includes flushing residualfluid in the intermediate transfer line into the adsorptive separationchamber 100 and 105 so that the residual fluid has a greater dwell timewithin the chamber 100 and 105 before being subsequently withdrawn thanit otherwise would if the flow rate was constant during the step-timeinterval or if the second flow rate was greater than the first flowrate.

According to another aspect, the second flow rate is higher than thefirst flow rate so that a greater volume of fluid is flushed during thesecond portion of the step-time interval than during the first portionof the step-time interval. The process according to this aspect may beparticularly useful where residual fluid is being flushed away from theadsorptive separation chamber 100 and 105 with flushing fluid withdrawnfrom the adsorptive separation chamber 100 and 105. In this regard, theflushing fluid is provided a greater dwell time within the adsorptiveseparation chamber than when a constant flow rate is used or when thefirst flow rate is greater than the second flow rate. Thisadvantageously provides for greater separation of components in theflushing fluid so that the flushing fluid will be more similar incomposition than a subsequent stream withdrawn from or introduced intothe adsorptive separation chamber 100 and 105.

Turning to more of the particulars, the following examples generallyinclude a process wherein a feed stream 5 and a desorbent stream 10 areintroduced into different ports 25 via different transfer lines of theadsorptive separation chamber 100 and 105. An extract stream 15 and araffinate stream 20 are withdrawn through two other ports 25 via twodifferent transfer lines of the adsorptive separation chamber 100 and105. According to one aspect, as illustrated for example in FIG. 7, aprimary flush in 720 includes flushing an intermediate transfer line 715between a transfer line currently occupied by the feed stream 5 during astep and a transfer line occupied by the extract stream 15 during thestep. The residual fluid in the transfer line 715 may include primarilyresidual feed fluid. The process according to this aspect includesflushing the transfer line 715 at a higher first volumetric flow rateduring the first portion of the step-time interval than a secondvolumetric flow rate during the second portion of the step-timeinterval. In this manner, a greater volume of the residual feed fluid isflushed into the adsorptive separation chamber 100 or 105 during theinitial first portion of the step-time interval than during thesubsequent second portion. In this regard, the residual feed fluidflushed into the adsorptive separation chamber 100 or 105 is providedgreater dwell time in the adsorptive separation chamber 100 and 105 andaccess to the adsorbent therein for separation of the non-preferentiallyadsorbed component prior to withdrawal thereof through the extractstream 15 in a subsequent step. According to another aspect, the processincludes a primary flush out 710 which includes flushing an intermediatetransfer line 705 away from the adsorptive separation chamber 100 or 105with fluid withdrawn from the chamber as described previously. In oneexample, the process includes flushing the transfer line 705, which mayinclude residual extract fluid from being previously occupied by theextract stream, at a first volumetric flow rate during a first portionof the step-time interval that is less than a second volumetric flowrate during a second subsequent portion of the step-time interval. Inthis manner, the flushing fluid withdrawn from the desorption zone 60may include fluid similar in composition to the desorbent stream 10. Theprocess may include flushing the residual extract fluid from theintermediate transfer line 705 to the intermediate transfer line 715 toflush the residual feed fluid in the intermediate extract stream 715into the purification zone 55. In one approach, the process includesflushing the fluid at a first flow rate at the first portion of thestep-time interval that is greater than the second flow rate during thesecond portion of the step-time interval so that a greater volume of theresidual feed fluid is introduced into the purification zone 55 duringan earlier portion of the step-time interval so that more separation ofthe feed fluid can be achieved in the purification zone 55 prior to theextract stream 15 subsequently arriving at and being withdrawn throughthe intermediate transfer line 715 to increase the purity of the extractstream.

Similarly, with reference briefly to FIG. 6 as described previously, theprocess may instead include a secondary flush 605 that includes flushingthe intermediate transfer line 610 and directing the residual fluidflushed therefrom to a downstream separation apparatus, including in oneexample, an extract separation column 175 to separate the preferentiallyadsorbed component from the residual extract fluid in the intermediatetransfer line 610. The process according to this aspect may includeflushing the intermediate transfer line 610 at a first volumetric flowrate during a first portion of the step-time interval that is less thana second volumetric flow rate during a second subsequent portion of thestep-time interval. In this manner, the flushing fluid withdrawn fromthe desorption zone 60 may include fluid similar in composition to thedesorbent stream 10.

According to another aspect, an intermediate transfer line 725 may beflushed with flushing fluid to introduce residual fluid in theintermediate transfer line into the purification zone 55. In accordancewith this aspect, the process may include flushing the intermediatetransfer line 725 at a first flow rate during a first portion of thestep-time interval that is greater than a second flow rate during asubsequent second portion of the step-time interval so that a greatervolume of the residual fluid in the transfer line 725 is flushed intothe purification zone 55 during the first portion of the step-timeinterval than during the second portion. In this manner, the residualfluid will be present in the purification zone for a longer dwell timefor separation of components therein prior to being withdrawn by theextract stream 15 when arrives at the intermediate transfer line 725 ina subsequent step.

In another aspect, an intermediate transfer line 735 may be flushed witha flushing fluid away from the adsorptive separation chamber 100 or 105to remove residual fluid therefrom. In one approach, the intermediatetransfer line includes residual raffinate from the raffinate stream 20that occupied the intermediate transfer line 735 during a previous stepof the cycle. In accordance with this aspect, the process includesflushing the intermediate transfer line 735 with flushing fluidwithdrawn from the adsorption zone 50 at a first flow rate during afirst portion of the step-time interval that is less than a secondportion of the step-time interval. In this manner, the flushing fluidwill be present in the adsorptive separation chamber 100 or 105 for agreater amount of time prior to being withdrawn through the intermediatetransfer line for flushing the residual feed fluid therefrom.Accordingly, the flushing fluid from the adsorption zone 55 will have asimilar composition to the feed stream and will include less of thenon-preferentially adsorbed component of the raffinate stream. Afterflushing the intermediate transfer line, the flushing fluid will remaintherein as residual fluid that will be introduced with the feed stream 5when the feed stream 5 is introduced through the intermediate transferline 735 during a subsequent step to reduce contamination of the feedstream by an excess of non-preferentially adsorbed component.

Turning to FIGS. 1, 4, and 5, according to various aspects as describedpreviously, the intermediate transfer lines 45, 415, or 510 may beflushed away from the adsorptive separation chamber 100 or 105 to removeresidual fluid therefrom. The intermediate transfer lines 45, 415, or510 may be flushed by withdrawing flushing fluid from the purificationzone 55 into the intermediate transfer line to displace the residualfluid away from the adsorptive separation chamber 100 or 105 and willsubsequently be filled with residual flushing fluid from thepurification zone 55. According to one aspect, the process includesflushing the intermediate transfer line 45, 415, or 510 at a first flowrate during a first portion of the step-time interval and at a secondflow rate that is greater than the first flow rate during a subsequentsecond portion of the step-time interval. In this manner, the flushingfluid is provided with additional time in the purification zone 55 andaccess to the adsorbent therein for separation of the non-preferentiallyadsorbed component so that when the purification zone fluid is withdrawnfor flushing the intermediate transfer line 45, 415, or 510, it will besimilar in composition to the extract stream 15 that will be withdrawntherethrough during a subsequent step. The process according to thisaspect advantageously reduces the amount of the non-preferentiallyadsorbed component that remains in residual fluid within theintermediate transfer line 45, 405, or 510 that would otherwisecontaminate the extract stream 15 during withdrawal therethrough,thereby increasing the purity of the extract stream 15. In one approach,as described previously, the intermediate transfer line 415 is incommunication with the feed stream transfer line so that the residualfluid flushed from the intermediate transfer line is combined with thefeed stream 5. In another approach, as described above, the intermediatetransfer line 510 is in communication with another intermediate transferline 515 so that the residual fluid therein is flushed to the otherintermediate transfer line 515 to flush residual feed fluid therein intoa downstream portion of the purification zone 55.

In accordance with various aspects, the volumetric flow rate of thefluid through the transfer lines during dynamic flushing thereof may becontrolled using valving and a controller. The valving may beincorporated into transfer lines themselves to control or restrict thevolumetric flow rate of fluid flowing therethrough. A controller may beprovided for controlling the valves and the flow rate of the fluidthrough the transfer lines. The valving may also be incorporated inother locations within the system, for example on the downstream side ofa rotary valve 300 when a rotary valve is incorporated or in downstreamlines for transferring the fluid to downstream components of the system,for example the lines 15′ and 20′ for transferring fluid to the extractfractionation column 175 or the raffinate fractionation column 150,respectively.

In selecting an adsorbent for the present simulated-moving-bed process,the only limitation is the effectiveness of the particularadsorbent/desorbent combination in the desired separation. An importantcharacteristic of an adsorbent is the rate of exchange of the desorbentfor the extract component of the feed mixture materials or, in otherwords, the relative rate of desorption of the extract component. Thischaracteristic relates directly to the amount of desorbent material thatmust be employed in the process to recover the extract component fromthe adsorbent. Faster rates of exchange reduce the amount of desorbentmaterial needed to remove the extract component, and therefore, permit areduction in the operating cost of the process. With faster rates ofexchange, less desorbent material has to be pumped through the processand separated from the extract stream for reuse in the process.

The practice of the subject invention thus is not related to or limitedto the use of any particular adsorbent or adsorbent/desorbentcombination, as differing sieve/desorbent combinations are used fordifferent separations. The adsorbent may or may not be a zeolite.Examples of adsorbents which may be used in the process of thisinvention include nonzeolitic molecular sieves including carbon-basedmolecular sieves, silicalite and the crystalline aluminosilicatesmolecular sieves classified as X and Y zeolites. Details on thecomposition and synthesis of many of these microporous molecular sievesare provided in U.S. Pat. No. 4,793,984, which is incorporated hereinfor this teaching. Information on adsorbents may also be obtained fromU.S. Pat. Nos. 4,385,994; 4,605,492; 4,310,440; and 4,440,871.

In adsorptive separation processes, which generally are operatedcontinuously at substantially constant pressures and temperatures toinsure liquid phase, the desorbent material must be selected to satisfyseveral criteria. First, the desorbent material should displace anextract component from the adsorbent with reasonable mass flow rateswithout itself being so strongly adsorbed as to unduly prevent anextract component from displacing the desorbent material in a followingadsorption cycle. Expressed in terms of the selectivity, it is preferredthat the adsorbent be more selective for all of the extract componentswith respect to a raffinate component than it is for the desorbentmaterial with respect to a raffinate component. Secondly, desorbentmaterials must be compatible with the particular adsorbent and theparticular feed mixture. More specifically, they must not reduce ordestroy the capacity of the adsorbent or selectivity of the adsorbentfor an extract component with respect to a raffinate component.Additionally, desorbent materials should not chemically react with orcause a chemical reaction of either an extract component or a raffinatecomponent. Both the extract stream and the raffinate stream aretypically removed from the adsorbent void volume in admixture withdesorbent material and any chemical reaction involving a desorbentmaterial and an extract component or a raffinate component or both wouldcomplicate or prevent product recovery. The desorbent should also beeasily separated from the extract and raffinate components, as byfractionation. Finally, desorbent materials should be readily availableand reasonable in cost. The desorbent may include a heavy or lightdesorbent depending on the particular application. The terms heavy andlight are in reference to the boiling point of the desorbent relative tothe C8 aromatics, namely, ortho-, meta-, para-xylene and ethylbenzene.Those skilled in the art will appreciate that the designator “C8” refersto a compound comprising eight (8) carbon atoms. In certain embodiments,the heavy desorbent is selected from the group consisting ofpara-diethylbenzene, para-diisopropylbenzene, tetralin, and the like,and combinations thereof. In certain embodiments, toluene and the likecan be used as the light desorbent. The para-diethylbenzene (p-DEB) hasa higher boiling point than the C8 aromatic isomers and, as such, thep-DEB is the bottoms (i.e., heavy) product when separated from the C8isomers in a fractional distillation column. Similarly, toluene has alower boiling point than the C8 aromatic isomers and, as such, thetoluene is the overhead (i.e., light) product when separated from the C8isomers in a fractional distillation column. The p-DEB has become acommercial standard for use as a desorbent in separations ofpara-xylene.

Adsorption conditions in general include a temperature range of fromabout 20° to about 250° C., with from about 60° to about 200° C. beingpreferred for para-xylene separation. Adsorption conditions also includea pressure sufficient to maintain liquid phase, which may be from aboutatmospheric to 2 MPa. Desorption conditions generally include the samerange of temperatures and pressure as used for adsorption conditions.Different conditions may be preferred for other extract compounds.

The above description and examples are intended to be illustrative ofthe invention without limiting its scope. While there have beenillustrated and described particular embodiments of the presentinvention, it will be appreciated that numerous changes andmodifications will occur to those skilled in the art, and it is intendedin the appended claims to cover all those changes and modificationswhich fall within the true spirit and scope of the present invention.

1. A process for separating components in a feed stream by simulatedcountercurrent adsorptive separation comprising: introducing a feedstream, comprising at least one preferentially adsorbed component and atleast one non-preferentially adsorbed component, and a desorbent streaminto two different ports via two different corresponding transfer linesalong a multi-bed adsorptive separation chamber having a plurality ofbeds that are serially connected in fluid communication and comprising apredetermined number of spaced ports with corresponding transfer linesin fluid communication therewith for introducing and removing fluid intoand from the adsorptive separation chamber for a predetermined step-timeinterval and withdrawing an extract stream and raffinate stream throughtwo different ports of the multi-bed adsorptive separation chamber viatwo different corresponding transfer lines for the predeterminedstep-time interval; flushing an intermediate transfer line at first flowrate during a first portion of the step-time interval; and flushing theintermediate transfer line at as second different flow rate during asecond portion of the step-time interval so that a greater volume offluid is flushed from the intermediate transfer line during one of thefirst portion and the second portion of the step-time interval thanduring the other of the first portion and the second portion of thestep-time interval.
 2. The process of claim 1, wherein the flow rate isa stepped flow rate and the first flow rate is a first constant flowrate and the second flow rate is a second constant flow rate.
 3. Theprocess of claim 1, wherein one of the first flow rate and the secondflow rate is about zero and the other of the first flow rate and thesecond flow rate is greater than about zero.
 4. The process of claim 1,wherein at least a portion of one or both of the first flow rate and thesecond flow rate is ramped and changes over time.
 5. The process ofclaim 1, wherein flushing the intermediate transfer line includeswithdrawing flushing fluid from a portion of the adsorptive separationchamber adjacent to a corresponding port of the intermediate transferline into the intermediate transfer line to flush residual fluid in theintermediate transfer line away from the adsorptive separation chamberand the second flow rate is greater than the first flow rate so that theflushing fluid is similar in composition to a subsequent stream thatwill occupy the intermediate transfer line.
 6. The process of claim 1,wherein flushing the intermediate transfer line includes flushingresidual fluid within the intermediate transfer line into a portion ofthe adsorptive separation chamber adjacent to a corresponding port ofthe intermediate transfer line and the first flow rate is greater thanthe second flow rate so that components in the residual fluid may beseparated in the adsorptive separation chamber.
 7. The process of claim1, wherein first portion of the step-time interval is between about 5%and 95% of the step-time interval and the second portion of thestep-time interval is between about 5% and 95% of the step-timeinterval.
 8. The process of claim 1, wherein one of the first flow rateand second flow rate is about 50% to about 400% of the volume of theintermediate transfer line and associated valving per the correspondingone of the first and second portion of the step-time interval.
 9. Theprocess of claim 1, further comprising analyzing a composition of aflushing stream and flushing the intermediate transfer line at the firstflow rate when the composition of a component is at a firstpredetermined level and flushing at the second flow rate when thecomposition is at a second predetermined level.
 10. The process of claim1, wherein the intermediate transfer line has a corresponding port influid communication with a purification zone of the adsorptiveseparation chamber between the extract stream and the feed stream andflushing the intermediate transfer line includes flowing flushing fluidinto the intermediate transfer line toward the adsorptive separationchamber to displace a portion of residual fluid within the adsorptiveseparation chamber into the purification zone via a corresponding portof the purification zone; and wherein the first flow rate is greaterthan the second flow rate so that a greater volume of the residual fluidis flushed into the purification zone during the first portion of thestep-time interval than during the second portion of the step-timeinterval so that components in the residual fluid may be separated inthe purification zone.
 11. The process of claim 1, wherein theintermediate transfer line has a corresponding port in fluidcommunication with an adsorption zone of the adsorptive separationchamber between the raffinate stream and the extract stream and flushingthe intermediate transfer line includes withdrawing flushing fluid fromthe adsorption zone through the port and into the intermediate transferline to flush residual fluid therein away from the adsorptive separationchamber; and wherein the second flow rate is greater than the first flowrate so that a greater volume of the flushing fluid is withdrawn intothe intermediate transfer line during the second portion of thestep-time interval than during the first portion of the step-timeinterval to reduce an amount of a non-preferentially adsorbed componentin the intermediate transfer line when the feed stream is introducedtherethrough in a subsequent step.
 12. The process of claim 1, whereinthe intermediate transfer line has a corresponding port in fluidcommunication with a purification zone of the adsorptive separationchamber between the extract stream and the feed stream and flushing theintermediate transfer line includes withdrawing flushing fluid from thepurification zone through the port and into the intermediate transferline to flush residual fluid therein away from the adsorptive separationchamber; and wherein the second flow rate is greater than the first flowrate so that a greater volume of the flushing fluid is withdrawn intothe intermediate transfer line during the second portion of thestep-time interval than during the first portion of the step-timeinterval to reduce an amount of a non-preferentially adsorbed componentin the intermediate transfer line when the extract stream is withdrawntherethrough in a subsequent step.
 13. The process of claim 1, whereinthe intermediate transfer line has a corresponding port in fluidcommunication with a purification zone of the adsorptive separationchamber between the extract stream and the feed stream and flushing theintermediate transfer line includes withdrawing flushing fluid from thepurification zone through the port and into the intermediate transferline to flush residual fluid therein away from the adsorptive separationchamber and directing the residual fluid to the feed stream to becombined therewith; and monitoring a composition of the residual fluidand flushing fluid flushed through the intermediate transfer line andwherein the first portion of the step-time interval occurs when thecomposition includes a component present at a first predetermined leveland the second portion of the step-time interval occurs when thecomponent is present at a second predetermined level.
 14. A process forthe separation of components in a feed stream comprising at least onepreferentially adsorbed component and at least one non-preferentiallyadsorbed component by simulated countercurrent adsorptive separationcomprising: introducing a feed stream into a port of a multi-bedadsorptive separation chamber comprising a plurality of ports withcorresponding transfer lines via one transfer line in fluidcommunication with the port; and withdrawing an extract stream from theadsorptive separation chamber through the one transfer line, wherein theextract stream has a higher concentration of the preferentially adsorbedcomponent than the feed stream and a lower concentration of thenon-preferentially adsorbed component than the feed stream; introducinga desorbent stream into the port of the adsorptive separation chamber;withdrawing an extract stream from the adsorptive separation chamber,wherein a portion of each stream introduced and withdrawn through thetransfer line remains in the transfer line as residual fluid; andflushing the transfer line at first flow rate for a first portion of astep-time interval during which the transfer line is not occupied by oneof the feed stream, the extract stream, the desorbent stream, and theraffinate stream; and flushing the intermediate transfer line at assecond different flow rate for a second portion of the step-timeinterval so that a greater volume of fluid is flushed from the transferline during one portion of the step-time interval than the other portionof the step-time interval.
 15. The process of claim 14, wherein flushingthe transfer line includes withdrawing flushing fluid from a portion ofthe adsorptive separation chamber adjacent to the corresponding port ofthe transfer line into the transfer line to flush the residual fluid inthe transfer line away from the adsorptive separation chamber and thesecond flow rate is greater than the first flow rate so that theflushing fluid is similar in composition to a subsequent stream thatwill occupy the transfer line.
 16. The process of claim 14, whereinflushing the transfer line includes flushing residual fluid within thetransfer line into a portion of the adsorptive separation chamberadjacent to the corresponding port of the transfer line and the firstflow rate is greater than the second flow rate so that components in theresidual fluid may be separated in the adsorptive separation chamber.17. The process of claim 14, wherein the transfer line port is in fluidcommunication with a purification zone of the adsorptive separationchamber during the step-time interval between steps during which thefeed is introduced through the transfer line and the extract iswithdrawn through the transfer line and flushing the transfer lineincludes flowing flushing fluid into the transfer line toward theadsorptive separation chamber to displace at least a portion of theresidual fluid within the adsorptive separation chamber into thepurification zone via a corresponding port of the purification zone; andwherein the first flow rate is greater than the second flow rate so thata greater volume of the residual fluid is flushed into the purificationzone during the first portion of the step-time interval than during thesecond portion of the step-time interval so that components in theresidual fluid may be separated in the purification zone.
 18. Theprocess of claim 14, wherein the transfer line port is in fluidcommunication with an adsorption zone of the adsorptive separationchamber during the step-time interval between steps during which theraffinate stream is withdrawn through the transfer line and the feedstream is introduced through the transfer line and flushing the transferline includes withdrawing flushing fluid from the adsorption zonethrough the port and into the transfer line to flush residual fluidtherein away from the adsorptive separation chamber; and wherein thesecond flow rate is greater than the first flow rate so that a greatervolume of the flushing fluid is withdrawn into the transfer line duringthe second portion of the step-time interval than during the firstportion of the step-time interval to reduce an amount of anon-preferentially adsorbed component in the transfer line when the feedstream is introduced therethrough.
 19. The process of claim 14, whereinthe transfer line port is in fluid communication with a purificationzone of the adsorptive separation chamber during the step-time intervalbetween steps during which the extract stream is withdrawn through thetransfer line and the feed stream is introduced through the transferline and flushing the transfer line includes withdrawing flushing fluidfrom the purification zone through the port and into the transfer lineto flush residual fluid therein away from the adsorptive separationchamber; and wherein the second flow rate is greater than the first flowrate so that a greater volume of the flushing fluid is withdrawn intothe transfer line during the second portion of the step-time intervalthan during the first portion of the step-time interval to reduce anamount of a non-preferentially adsorbed component in the transfer linewhen the extract stream is withdrawn therethrough in a subsequent step.20. The process of claim 14, wherein the transfer line port is in fluidcommunication with a purification zone of the adsorptive separationchamber during the step-time interval between steps during which theextract stream is withdrawn through the transfer line and the desorbentstream is introduced through the transfer line and flushing the transferline includes withdrawing flushing fluid from the purification zonethrough the port and into the transfer line to flush residual fluidtherein away from the adsorptive separation chamber and directing theresidual fluid to the feed stream to be combined therewith; andmonitoring a composition of the residual fluid and flushing fluidflowing through the intermediate transfer line and wherein the firstportion of the step-time interval occurs when the composition includes acomponent present at a first predetermined level and the second portionof the step-time interval occurs when the component is present at asecond predetermined level.